Near-Field Ultrafast Nanoscopy of Energy Transport in Semiconductor Nanowires

Carrier distribution and dynamics in semiconductor materials often govern their physical properties that are critical to functionalities and performance in industrial applications. Conventional pump-probe microscopy has limited spatial resolution due to the optical diffraction. Recently, ultrafast infrared-terahertz nano-spectroscopy was developed through the integration of scanning near-field optical microscopy and pump-probe optics. However, given the limited photon energy, the efforts have been primarily focused on studying carrier dynamics in narrow bandgap semiconductors or graphene plasmons.<br/>Here, we first report near-field ultrafast optical nanoscopy in the visible-near-infrared spectral region to access the carrier dynamics in silicon, one of the most prevalent materials in current semiconductor technology. Our pump beam has a wavelength of 400 nm (3.1 eV), which is sufficient to excite carriers in common optoelectronic semiconductors, including silicon (bandgap of 1.12 eV) and GaAs (bandgap of 1.42 eV). By combining ultrafast nanoscale measurements and theoretical modeling, we unravel the local photocarrier recombination dynamics in silicon nanowires. Moreover, we demonstrate the spatial mapping of carrier lifetime in silicon with a sub-50 nm resolution. Our results provide the capability to probe carrier behaviors in nanoscale materials and devices. By utlizing this experimental platform, we can experimentally access the nonequilibrium regime of energy transport in the nanoscale. In this context, we will show spatially and temporally resolved probing of transient energy transport in nanowires and nanobeams.